1. Field of the Invention
The invention relates generally to methods to fabricate arrays of optical filters. More specifically, the invention relates to methods to fabricate arrays of narrow band pass filters for optical signal processing and optical communications.
2. Description of the Prior Art
In modern optical systems for signal processing or optical communications, filters are often used in order to select photons with the desired energies or light of the desired wavelengths. The central wavelengths of transmission and band widths of such filters are dependent upon the applications. For fiber optic communications, the desired wavelengths of light are within a band where the attenuation of light in the fibers is low. One of the important wavelength bands is from 1500 nm to 1560 nm. Certain systems involving optical fibers also operate at wavelengths in the range from 1240 nm to 1340 nm. In each of the above wavelength bands, it is desired to transmit more than one beam of light of different wavelengths.
A light beam at a particular wavelength is considered one channel. In practice, a light beam for one channel spreads over a very narrow range of wavelength. For high bit rate signal processing and optical communications, it is desirable to transmit as many channels as possible in a given band wavelengths. This is because the rate of data transmission when an optical fiber is used is directly proportional to the number of channels or wavelengths. To receive the signals carried by the fiber, it is necessary to select light in a given narrow wavelength range. This can be done by allowing light of a given narrow range to transmit through a filter. It can also be achieved by allowing light at wavelengths other than the narrow wavelength range to transmit through a filter and light at wavelengths within the given narrow range to reflect from the filter. The transmitted or reflected light within the narrow wavelength range is then processed and detected.
There are several methods which can be used to construct filters to separate light of different wavelengths. One of the methods is to use multilayer thin film optical filters. The principles of multilayer thin film optical filters are based on light interference and can be found in many textbooks (for example in Thin Films for Optical Systems edited by F. R. Flory, published by Marcel Dekker Inc., 1995). One of the possible structures is to deposit alternating thin film layers of two materials with different refractive indices. Each layer has a thickness of t.sub.i =.OMEGA./4n.sub.i, where .OMEGA. is the wavelength (in free space) of light to select or to transmit, n.sub.i is the refractive index of the material at the given wavelength. The transmission band width is determined by the number of thin film layers deposited. It should be noted that multilayer structures involving more than two materials may also be used.
Using this technology, optical filters with a narrow pass band (less than 1 nanometer, nm) have been fabricated and used in many optical devices and systems. The other way to separate light of different wavelengths is to use gratings. The gratings are fabricated on a flat substrate. When light is incident on the gratings, light of different wavelengths is diffracted in different directions. The spacing of adjacent grating grids is determined by the wavelengths of the light to be diffracted. The resolution of the grating devices can be made to be smaller than 1 nm. More recently, a technology has been developed and used to fabricate grating devices in an optical fiber directly. Such devices are called fiber gratings. In this method, a specially designed mask is placed on a fiber. Ultraviolet light is then allowed to go through the mask and incident onto the fiber. The refractive indices of the illuminated regions are altered to form fiber sections with periodic refractive index variation. When light is incident on the core of one end of the fiber, light within a specific wavelength range is reflected. The rest of the light is allowed to transmit through the fiber. Using this method, fiber gratings with very narrow reflection band have been fabricated and used.
As described before, in order to increase the data transmission rate in optical signal processing or optical communication systems, the data must be carried by light at different wavelengths concurrently in a fiber. The separation of light of different wavelengths is the most critical part in such applications. The device used to separate light of different wavelengths is called a wavelength division multiplexer (WDM). In some literatures, the device used to separate light of different wavelengths is also called a wavelength division demultiplexer. When the separation of wavelength .delta..OMEGA., of light beams propagating in a fiber is of the order of 0.5 to 5 nm, the device used for the separation is called a dense wavelength division multiplexer (DWDM). Conventional WDMs or DWDMs are constructed by combining several discrete filters, each with one specific transmission or one central reflection wavelength. For WDMs or DWDMs with thin film filters, each filter has to be tested and installed in a holder and aligned with input and output fibers. The testing, installing and alignment are rather time consuming. Further, the final WDMs or DWDMs may be quite bulky and quite expensive. From the above comments, it is evident that there is a need to provide an improved method to fabricate WDMs or DWDMs for optical signal processing and optical communication systems. The WDMs or DWDMs fabricated using the methods provided in this patent are suitable for optical signal processing systems and optical communication systems.